The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation, including radio waves, millimeter waves, microwaves, infrared, visible light, ultra-violet (INA and UVB), x-rays, and gamma rays. The Earth's ozone layer absorbs wavelengths up to approximately 286 nm, shielding human beings from exposure to electromagnetic radiation with the highest energy. However, humans are exposed to electromagnetic radiation having wavelengths above 286 nm. Most of this radiation falls within the human visual spectrum, which includes light having a wavelength ranging from approximately 400 nanometers (nm) to approximately 700 nm.
The human retina responds to visible light (400-700 nm). The shorter wavelengths of visible light pose the greatest hazard to human health because they inversely contain greater energy. In particular, blue light, ranging in wavelength from approximately 400 nm to approximately 500 nm, has been shown to be the portion of the visible spectrum that produces the most photochemical damage to animal retinal pigment epithelium (RPE) cells.
Cataracts and macular degeneration have been associated with photochemical damage to the intraocular lens and retina, respectively, resulting from blue light exposure. Blue light exposure has also been shown to accelerate proliferation of uveal melanoma cells. Recent research also supports the premise that short wavelength visible light (blue light) may contribute to age related macular degeneration (AMD).
The human retina includes multiple layers. These layers, listed in order from the first exposed to any light entering the eye to the deepest, include:
1) Nerve Fiber Layer
2) Ganglion Cells
3) Inner Plexiform Layer
4) Bipolar and Horizontal Cells
5) Outer Plexiform Layer
6) Photoreceptors (Rods and Cones)
7) Retinal Pigment Epithelium (RPE)
8) Bruch's Membrane
9) Choroid
When light is absorbed by the human eye's photoreceptor cells, (rods and cones) the cells bleach and become unreceptive until they recover. This recovery process is a metabolic process referred to as the “visual cycle.” Absorption of blue light reverses this process prematurely, increasing the risk of oxidative damage. This reversal leads to the buildup of lipofuscin in the RPE layer of the eye. Excessive amounts of lipofuscin lead to the formation of extracellular aggregates termed “drusen” between Bruch's membrane and the RPE of the eye.
Over the course of a person's life, metabolic waste byproducts accumulate within the RPE layer of the eye due to the interaction of light with the retina. Metabolic waste byproducts include certain fluorophores, such as lipofuscin constituent A2E. As this metabolic waste accumulates in the RPE layer of the eye, the body's physiological ability to metabolize waste diminishes, and blue light stimulus causes drusen to be formed in the RPE layer. It is believed that the drusen further interfere with the normal physiology/metabolic activity, contributing to AMD. AMD is the leading cause of irreversible severe visual acuity loss in the United States and Western World. The burden of AMD is expected to increase dramatically in the next 20 years because of the projected shift in population and the overall increase in the number of ageing individuals.
Drusen hinder or block the RPE layer from providing the proper nutrients to the photoreceptors, which leads to damage or even death of these cells. To further complicate this process, it appears that when lipofuscin absorbs blue light in high quantities it becomes toxic, causing further damage and/or death of the RPE cells. It is believed that the lipofuscin constituent A2E is at least partly responsible for the short-wavelength sensitivity of RPE cells. Lipofuscin chromophore A2E exhibits a maximum absorption of approximately 430 nm. The photochemical events resulting from the excitation of A2E can lead to cell death.
From a theoretical perspective, the following events appear to take place in the eye: (1) starting from infancy and throughout life, waste buildup, including buildup of lipofuscin, occurs within the RPE; (2) retinal metabolic activity and the eye's ability to deal with this waste typically diminishes with age; (3) macular pigment typically decreases with age, thus filtering out less blue light; (4) blue light causes the accumulating lipofuscin to become toxic, damaging pigment epithelial cells.
The lighting and vision care industries have standards as to human vision exposure to UVA and UVB radiation. Surprisingly, no such standard is in place with regard to blue light. For example, in the common fluorescent tubes available today, the glass envelope mostly blocks ultra-violet light but blue light is transmitted with little attenuation. In some cases, the envelope is designed to have enhanced transmission in the blue region of the spectrum. Such artificial sources of light hazard may also cause eye damage.
With a goal of protecting eyes from the potentially harmful effects of blue light, eyewear (e.g., sunglasses, spectacles, goggles, and contact lenses) configured to block blue light has been evaluated. Such eyewear typically employs a yellow dye or pigment (e.g., BPI Filter Vision 450 or BPI Diamond Dye 500) that absorbs incident blue light. As a result, such eyewear typically includes yellow tinted lenses that completely (or nearly completely) block light below a threshold wavelength (e.g., below 500 nm), while also reducing light exposure at longer wavelengths.
However, such eyewear has significant drawbacks for the user. In particular, blue blocking ophthalmic systems may be cosmetically unappealing because of a yellow or amber tint that is produced in lenses by blue blocking. To many people, the appearance of this yellow or amber tint may be undesirable cosmetically. Moreover, the tint may interfere with the normal color perception of a lens user, making it difficult, for example, to correctly perceive the color of a traffic light or sign.
Efforts have been made to compensate for the yellowing effect of conventional blue blocking filters. For example, blue blocking lenses have been treated with additional dyes, such as blue, red or green dyes, to offset the yellowing effect. The treatment causes the additional dyes to become intermixed with the original blue blocking dyes. However, while this technique may reduce yellow in a blue blocked lens, intermixing of the dyes may reduce the effectiveness of the blue blocking by allowing more of the blue light spectrum through. Moreover, these conventional techniques undesirably reduce the overall transmission of light wavelengths other than blue light wavelengths. This unwanted reduction may in turn result in reduced visual acuity for a lens user.
Conventional blue-blocking also reduces visible transmission, which in turn stimulates dilation of the pupil. Dilation of the pupil increases the flux of tight to the internal eye structures including the intraocular lens and retina. Since the radiant flux to these structures increases as the square of the pupil diameter, a lens that blocks half of the blue light but, with reduced visible transmission, relaxes the pupil from 2 mm to 3 mm diameter, will actually increase the dose of blue photons to the retina by 12.5%. Protection of the retina from phototoxic light depends on the amount of this light that impinges on the retina, which depends on the transmission properties of the ocular media and also on the dynamic aperture of the pupil.
Another problem with conventional blue-blocking is that it can degrade night vision. Blue light is more important for low-fight level or scotopic vision than for bright light or photopic vision, a result which is expressed quantitatively in the luminous sensitivity spectra for scotopic and photopic vision. Accordingly, blue-blocking eyewear that completely (or nearly completely) blocks incident light below a threshold wavelength (e.g., below 500 nm) can significantly impair night vision.
In addition, blue light is known to impact circadian rhythms. Melatonin (N-acetyl-5-methoxytryptamine) is a hormone secreted by the pineal gland. Melatonin, in part, regulates the sleep-wake cycle by chemically causing drowsiness and lowering the body temperature. Blue light having a wavelength of 460 to 480 nm suppresses melatonin production. Accordingly, ensuring proper levels of blue light throughout the day can be important for maintaining acceptable circadian rhythms.
Accordingly, there is a need for materials that can mitigate the harmful effects of blue light while maintaining acceptable photopic vision, scotopic vision, color vision, and circadian rhythms.